Recent reports have recognized that the occurrence, fate, and treatment of Pharmaceutical and Personal Care Products (PPCPs) in the environment are of increasing global concern. These so called “emerging contaminants” were actually first identified in the early 1980s and include a wide rage of chemicals such as analgesics (naproxen, acetaminophen, and ibuprofen), antibiotics (ciproflaxin, and triclosan), antihyperlipidemics (clofibric acid), and other compounds (bisphenol, salicylic acid, and carbamazepine). During decades, they have shown to be persistent in the environment probably due to long-shelf life formulations associated with product demand of a continuously growing population. Moreover, traditional water treatment methods are not suitable for the removal of PPCPs resulting in surface water concentrations in the parts-per-trillion (ppt) range. Although the ultimate impact of PPCPs on population health is still not fully understood, the removal of these contaminants from our potable water and aquatic systems is an important goal for worldwide government agencies and researchers.
Most PPCPs are polar compounds with acidic or basic functionalities that occur at low concentration levels in water sources. These characteristics generate unique challenges for their removal using traditional water treatment methods. Various sorbents have been proposed for the removal of a selected list of PPCPs at low concentrations. Examples of these include activated carbon, membranes, and polymers. In general, the corresponding uptake capacities of many of these materials showed to be highly dependent on pH conditions and specific interactions such as van der Waals, electrostatic, and dipole-dipole. Another study showed that the adsorption performance of silica, g-alumina and Parapak-P (non-polar organic medium) on selected PPCPs was also highly dependent on pH conditions, ionic level of the pharmaceutical, and the nature of the surface charge of the sorbents. To achieve better selectivity at the present and near future concentration levels it is critical to start considering the bottom-up design of surfaces with better specific interactions, tailored to remove specific families of PPCPs.
Mesoporous silica are good candidates for the design of PPCP selective sorbents due to their large surface areas, tunable pore structure and well understood surface chemistry. Since their discovery in 1992 mesoporous silicas from the M41S family have demonstrated their potential in applications like heavy metal and organic contaminants removal, catalysis, chromatographic separation, and controlled drug delivery. A recent report documents the use of nickel grafted on mesoporous support as a potential sorbent for the removal of Naproxen from water at ppm concentration levels. However, it was concluded that supports with better stability should be considered in order to increase the amount of metal incorporated onto the surface without compromising the structural and porosimetry properties. In addition, based on ab-initio calculations, the report suggested that the adsorption process could be described as a complexation mechanism between the metal center and the Naproxen molecule.
One possible way of reducing the grafting effects on the overall materials textural properties of mesoporous silica is by employing substrates that display dense pore walls. SBA-15 mesoporous silica are known to have thick pore walls (3.1-6.4 nm) and considerable pore diameters (4.6-30 nm), characteristics that are essential to facilitate the grafting while providing the required pore volume for the removal of large sorbates. It should be mentioned that recently it has been reported the use of unmodified mesoporous SBA-15 for the removal of a set of PPCPs. Although they showed that the materials performance was greatly dependent on pH, no attempt to modify the solid surface chemistry was made.